As a process for producing an alcohol, a method of obtaining a saturated alcohol by hydrogenating an aldehyde and purifying the product is hitherto known and has been commercialized worldwide. For example, with regard to saturated aldehydes, butyraldehyde can be hydrogenated to afford butanol and nonyl aldehyde can be hydrogenated to afford nonanol, and with regard to unsaturated aldehyde, 2-ethylhexenal can be hydrogenated to afford 2-ethylhexanol, 2-propylheptenal can be hydrogenated to afford 2-propylheptanol, and decenal can be hydrogenated to afford decanol.
As the mode of the hydrogenation reaction, it is common to use a reactor, inside of which is usually packed with a nickel-based or cooper-based solid hydrogenation catalyst. There are a mode where a starting aldehyde is vaporized to carrying out the reaction in a vapor phase and a mode where a starting aldehyde is introduced as a liquid into a reactor to carrying out in a liquid phase.
However, regardless of the catalyst species and the reaction mode of vapor phase/liquid phase, there are problems that esterification, acetalization, etherification, and the like occur as undesirable side reactions to lower the selectivity of the reaction in any of the conventional reaction processes and also a satisfactory product alcohol cannot be obtained unless these by-products are separated/removed by distillation operation or the like for purification in subsequent step(s).
As the purification/distillation method of the above crude alcohol, the following methods have been proposed, for example.
The first method is a method of separating low-boiling products in a first column, then separating the alcohol from high-boiling products by distillation to obtain the alcohol product as a distillate from the column top by controlling column top pressure in the second column, and recovering useful products in high-boiling components by controlling the column top pressure in the third column (3-column mode).
Specifically, in Patent Document 1 (JP-B-49-11365), there is described a method wherein purified 2-ethylhexanol is obtained by operating the second column under the conditions of a top pressure of 200 to 800 mmHg and an alcohol content in the bottom of 50 wt % or more and by operating the third column under the condition of a top pressure of 70 to 300 mmHg in the above 3-column mode.
Moreover, there is also known a method wherein the first column is carried out in two steps in the above 3-column mode (4-column mode), i.e., a method wherein low-boiling products are separated by distillation in the first column; then the product alcohol is distilled in the second column; the bottom liquid is further treated to concentrate and separate high-boiling products from the bottom in the third column and then an effective component is recovered by distillation; and the low-boiling products separated in the first column are further concentrated and separated by distillation in the fourth column and an effective component is recovered from the bottom.
Additionally, in order to avoid contamination of the alcohol product distilled from the column top with low-boiling products formed by thermal decomposition of the high-boiling components, particularly acetal components, ether components, and the like in the bottom liquid in the above second column from which the product alcohol is obtained, there is also known a method wherein high-boiling components are separated in the first column, a fraction containing low-boiling components and the alcohol and containing substantially no high-boiling components is distilled from the column top, the fraction is fed to the second column, the low-boiling components are separated from the alcohol, and a fraction containing the low-boiling components as main components is distilled from the column top, while purified alcohol is distilled as a side cut (cf. Patent Document 2).
Furthermore, in the above 2-column mode, there is disclosed a method wherein the high-boiling components are positively thermally cracked and the high-boiling components are recovered as effective component(s) by maintaining the bottom temperature to the value calculated from a prescribed equation or a higher value thereof and the concentration of the high-boiling components in the bottom liquid to 30 wt % or more in the first column from which the high-boiling components are separated (cf. Patent Document 3).
On the other hand, in general, since the product alcohol is frequently used mainly as a plasticizer for resins such as vinyl chloride, an extremely high purity is required and a little coloring, i.e., a little degree of coloring in the sulfuric acid coloring test is required, the test being conducted by heating a sample together with sulfuric acid and then measuring the degree of coloring.
As a component extremely strongly affecting the above sulfuric acid coloring test or the like, an aldehyde may be mentioned. This is because an aldehyde is an unsaturated hydrocarbon. Therefore, the concentration of the aldehyde contained in the product alcohol is one of the most important items for quality of the product alcohol and is desirably reduced.
However, relatively a large amount of the aldehyde is contained in the product alcohol obtained by any of the above precedent methods and thus the product is not thoroughly satisfactory. However, in the above precedent technologies, the concentration of the aldehyde contained in the product alcohol is not at all focused and hence no method for lowering the concentration is disclosed. This may be attributed to the fact that the following can be easily supposed for those skilled in the art when they consider based on common knowledge of chemical engineering without particular disclosed technology.
That is, as methods for reducing the concentration of the aldehyde contained in the product alcohol, there may be considered 1) a method of reducing the amount of unreacted aldehyde to be introduced into the purification system by increasing the conversion rate of the aldehyde into the alcohol in the hydrogenation reaction; 2) a method of increasing the degree of separation of the aldehyde as a low-boiling component by increase of the plate number of the distillation column, increase of reflux ratio, or the like in the step of separating low-boiling components in the purification system; and the like method.
Also, in the actual commercial running, it is supposed that the maintenance of the quality of the product alcohol, i.e., the maintenance of the concentration of the aldehyde contained to a standard value or a lower value thereof may be attempted by the following methods: reduction of the amount of unreacted aldehyde to be introduced into the purification system by changing running conditions such as reaction temperature and the like to suppress the decrease of conversion rate of the aldehyde, the decrease being induced with the decrease of activity of the hydrogenation catalyst with the passage of time (i.e., the method of the above 1)) or increase of the separation efficiency of the aldehyde by increasing the reflux amount or the distillate amount in the low-boiling component-separating column in the purification system (i.e., the method of the above 2)).
However, currently it is very difficult to obtain an alcohol having a low aldehyde concentration.
On the other hand, in the purification/distillation of C3-C10 alcohols, it is confirmed that corresponding aldehydes are formed by heat load at the bottom part of the distillation column and a method of distillation in the presence of an alkali metal hydroxide is disclosed as a suppressing means (cf. Patent Document 4). However, in this method, there is a problem that an additional facility for adding the alkali metal hydroxide is necessary and it is impossible to deny a possibility of contamination of the additive into the product alcohol                [Patent Document 1] JP-B-49-11365        [Patent Document 2] JP-A-6-122638        [Patent Document 3] JP-A-7-278032        [Patent Document 4] JP-T-11-500437        